LASER-ABLATION U Th Pb IN SITU DATING OF ZIRCON AND ALLANITE: AN EXAMPLE FROM THE OCTOBER HARBOUR GRANITE, CENTRAL COASTAL LABRADOR, CANADA

Abstract

U-Th-Pb geochronology based on data acquired by laser-ablation microprobe -inductively coupled plasma -mass spectrometry (LAM-ICP-MS) has been applied to in situ dating of allanite and zircon in a single section of the October Harbour granite, Labrador. This is the first report of Th-Pb dating of allanite by the LAM-ICP-MS method, confirmed by zircon ages from the same section. Crystals as small as 20 ϫ 20 m have been analyzed in a standard polished section 100 m thick. Zircon crystals were examined using back-scattered electron images to identify features such as inherited cores, overgrowths and oscillatory growth. In this study, 45 LAM-ICP-MS analyses were carried out on 35 crystals of zircon. The U-Th-Pb zircon data suggest that the age of the granite is 1657 ± 10 Ma (MSWD = 0.77, weighted mean 207 Pb/ 206 Pb ages from 25 concordant points). The data also indicate 1800-1850 Ma inheritance and patterns of multiple loss of lead in many grains. The allanite occurs as large euhedral crystals; they show elevated U and Th contents. Twenty-six analyses of two such crystals of allanite gave sixteen concordant points, obtained mainly from the low-Th core of a single large crystal, and a weighted mean Th age of 1635 ± 17 Ma (MSWD = 2.1). Although complex Pb-loss is also recorded in portions of the allanite crystals analyzed, no evidence of inheritance was recorded. The age of the October Harbour granite is constrained to be ca. 1640-1660 Ma; it thus is significantly younger than the previously defined conventional multi-grain U-Pb zircon age of 1719 ± 3 Ma attributed to the postulated parental intrusive complex, the Strawberry intrusive suite. The Strawberry suite thus may not be a single ca. 1720 Ma suite of rocks, but a more complex composite unit represented by rocks of widely differing ages. Mots-clés: ablation au laser, analyses ICP-MS, geochronologie U-Th-Pb, datation in situ, zircon, allanite, granite de October Harbour, Labrador.

Laser assisted plasma spectrochemistry: laser ablation

“…[31][32][33][34][35][36][37][38][39][40] It is important to point out that the occurrence of fractionation does not preclude the use of laserablation sampling for accurate chemical analysis: numerous successful applications have been demonstrated. [41][42][43][44][45][46][47][48][49][50] Fractionation is a function of laser beam properties (irradiance, pulse duration, wavelength). The wavelength effect on fractionation is of great interest.…”

Insights into the Acadian orogeny, New England Appalachians: a provenance study of the Carrabassett and Kittery formations, Maine

“…Raw data were corrected for electron multiplier dead time (20 ns) and processed off-line using an Excel spreadsheet program (LAMDATE) to integrate signals from each sequential set of 10 sweeps; the method follows that of Kosler et al (2002) 235 U since it was not acquired with other isotopes due to its low natural abundance. Aspiration of the tracer solution allowed for a real-time instrument mass bias correction using known isotopic ratios of the tracer solution measured while the sample was ablated; this technique is largely independent of matrix effects that can variably influence measured isotopic ratios and hence the resulting ages (Kosler et al 2002;Cox et al 2003). The amount of common Pb present in the analyzed zircon grains was insignificant relative to the con- tent of radiogenic Pb and therefore, no common Pb correction was applied to the data.…”

Origine et développement de litages dans des roches de composition granitique /

“…Didier & Barbarin, 1991;. Seuls de très rares cas font état de formation de litage dans des roches granitiques qui ont elles aussi subi de fortes déformations Par la suite les données obtenues sont corrigées par différents processus afin d'obtenir des rapports plus proches de la réalité (Cox et al, 2003) ; les corrections sont les suivantes :…”